TCP/IP is a set of protocols developed to allow cooperating
computers to share resources across a network. It was developed by a community
of researchers centered around the ARPAnet. Certainly theARPAnet is the best-known
TCP/IP network. However as of June, 87, at least 130 different vendors had products
that support TCP/IP, and thousands of networks of all kinds use it. First some
basic definitions. The most accurate name for the set of protocols we are describing
is the "Internet protocol suite". TCP and IP are two of the protocols
in this suite. Because TCP and IP are the best known of the protocols, it has
become common to use the term TCP/IP or IP/TCP to refer to the whole family. It
is probably not worth fighting this habit. However this can lead to some oddities.
For example, I find myself talking about NFS as being based on TCP/IP, even though
it doesn't use TCP at all. The Internet is a
collection of networks, including the Arpanet, NSFnet, regional networks such
as NYsernet, local networks at a number of University and research institutions,
and a number of military networks. The term "Internet" applies to this
entire set of networks.

The subset of them that is managed by the Department of
Defense is referred to as the "DDN" (Defense Data Network). This includes
some research-oriented networks, such as the Arpanet, as well as more strictly
military ones. (Because much of the funding for Internet and DDN can sometimes
seem equivalent.) All of these networks are connected to each other. Users can
send messages from any of them to any other, except where there are security or
other policy restrictions on access. Officially speaking, the Internet protocol
documents are simply standards adopted by the Internet community for its own use.
More recently, the Department of Defense issued a MILSPEC definition of TCP/IP.
This was intended to be a more formal definition, appropriate for use in purchasing
specifications. However most of the TCP/IP community continues to use the Internet
standards.